The present invention relates to apparatus and methods for treating contaminated soils, particularly those containing hydrocarbon products and hydrocarbon chemicals, such as PCBs, and particularly relates to apparatus and methods for remediating hydrocarbon-contaminated soils in a thermally efficient, environmentally compatible and safe manner.
Soils are frequently contaminated with hydrocarbon products and this constitutes a highly significant and major pollution problem. The contaminants may range from gasoline through heavy hydrocarbon products and hydrocarbon chemicals, such as PCBs. Various efforts have been directed to remediating the soil and one of the most effective is to thermally treat the soil. However, high cost is an inhibiting factor and, in many cases, is the result of inefficiently designed equipment and limited equipment capacities. For example, a major factor affecting the cost is fuel efficiency, as well as the downstream treatment of the residual gaseous components driven off from the soil. Thermal efficiency is disregarded in many systems. For example, water spray quench systems are frequently used for treating exhaust gas streams, often without regard to heat recovery. Heat recovery in high temperature fume incineration is also frequently ignored. Consequently, the cost for clean-up of contaminated soil ranges typically from $30 a ton to well over $300 per ton, depending on the level of contamination, type of contaminant, type of soil in which the contaminant exists, and overall quantity of the contaminant.
In these prior systems, the basic process for cleaning the soil is to expose it to high temperatures whereby the contaminant is volatilized and subsequently oxidized or processed in a reducing environment to leave a carbon char material in the soil. The temperatures at which the soils must be processed can vary substantially from as low as 300.degree. F. discharge temperature on the soil to over 1,000.degree. F. in order to obtain satisfactory low levels of total residual petroleum hydrocarbons. With these wide-ranging temperatures necessary to clean up a wide variety of contaminants, it is essential to design a remediation system which, not only effectively removes the contaminants, but does so in a thermally and, hence, fuel efficient manner.
In accordance with the present invention, a thermally and fuel efficient system for cleaning a wide variety of soils contaminated with different hydrocarbon products is provided. The system includes a counterflow dryer roaster wherein contaminated soil is supplied the elevated end of an inclined drum for flow toward the opposite end, and at which end a burner is mounted. The exhaust gases from the burner and the residual contaminated dust and gases driven from the soil are delivered through an outlet at the upstream inclined end of the dryer to a dust collector, preferably a cyclone. The counterflow drum dryer has internal flights to provide heavy veiling of the soil and rapid convective heat transfer from the hot gas stream to the soil. The drum also has a high-temperature refractory zone downstream from the soil inlet and adjacent the burner for achieving very high temperatures. Consequently, the soil can be processed at wide-ranging temperatures, enabling remediation of soils contaminated with different types of hydrocarbons.
The soil is discharged from the dryer roaster into a rotary cooler for cooling the remediated soil prior to discharge. The cooler has a water inlet which creates steam in the cooler. The dust from the separator is passed through primary and secondary baghouses, where hydrocarbons thereon are volatilized by heat transfer from gases discharged from a thermal oxidizer. The clean dust is then passed into the rotary cooler for thorough mixing with the soil discharged from the dryer roaster. The steam from the rotary cooler is blended with hot gases from a stack and the thermal oxidizer discharge for heat recovery purposes and to elevate the temperature of the gases in the secondary baghouse above the dewpoint. The clean soil is, of course, discharged from the rotary cooler.
The exhaust gases after dust separation in the primary baghouse are supplied to a heat exchanger which also receives the high-temperature exhaust from the thermal oxidizer. In the thermal oxidizer, the exhaust gases are heated to a set point temperature, e.g., 1200.degree. F. to 1600.degree. F., to fully destroy residual hydrocarbons in the gas stream. The discharge from the thermal oxidizer passes through the heat exchanger and into a stack for release. A portion of the discharged gas, however, is diverted for passage through the primary baghouse and into the dust collector to heat the dust collected in these units sufficiently to volatilize any residual hydrocarbon, leaving the dust clean. Additionally, the discharge from the thermal oxidizer or from the stack is blended with the steam from the rotary cooler in the secondary baghouse to raise the temperature of the gases in the secondary baghouse above the dewpoint temperature, e.g., 200.degree. F. to 250.degree. F. Thus, any dust from the treated soil in the rotary cooler is heated in the secondary baghouse by heat recovered from the thermal oxidizer and its temperature is therefore elevated sufficiently to keep these materials above the dewpoint temperature so that the baghouse can operate properly. Consequently, by the counterflow design of the dryer roaster and the high temperature gases from the thermal oxidizer, the very high temperatures necessary to the volatilization of a large range of commonly anticipated contaminants can be achieved on the soil, with relatively low temperatures in the exhaust gas train from the dryer and stack, whereby minimization of costs of the units is achieved. Also, by coupling the discharge from the thermal oxidizer in heat exchange relation with the gases going to the thermal oxidizer, significant amounts of energy from the thermal oxidizer is recovered.
Also unique to the present system is a dust oven disposed adjacent the bottom of the baghouses and the collector for volatilizing residual hydrocarbons on the dust. Dust collected by the separator is conveyed into the primary baghouse by a screw conveyor. The screw conveyor is internal to the dust oven. The outside tubular housing of the screw is perforated so that the collected dust will be discharged along its length inside the dust oven. The dust is picked up by the gases flowing through the oven, exiting at the bottom and in the process are elevated to a high temperature to volatilize any remaining hydrocarbons. In this oven, dust collected by those units is conveyed into the secondary baghouse and into the cooler by a screw conveyor. Around the conveyor from the dust collector, there is provided a hot gas oven comprised of an elongated tube designed to effect efficient heat transfer with the dust falling to the bottom of the hoppers toward the screw conveyor in the primary baghouse. The heat transfer is provided by discharging exhaust gases (1200.degree. F. to 1600.degree. F.) from the thermal oxidizer into the tube wherein heat energy is transferred by conduction through the conveyor housing to the dust as the dust flows downwardly over and about it. These hot gases also exhaust the oven tube in a downward direction for flow upwardly countercurrently to the downward flow of dust from the primary baghouse into the screw conveyor and for additional heat transfer to the dust from the primary baghouse. Any residual hydrocarbons on the dust are thus volatilized and carried with the gases through the primary baghouse to the thermal oxidizer. The bottom panels of the dust oven are hinged, so that the clearance between the side walls and these panels can be adjusted down the length of the baghouse to provide for adjustment of the heat transfer from the gases to the downwardly flowing dust.
In a preferred embodiment according to the present invention, there is provided a method for remediating contaminated soils comprising the steps of heating the soil in a rotating drum to volatilize the contaminant, flowing particulate-laden gases from the drum through a first particle separator for separation into a first exhaust gas stream and a first particle stream, passing the heated soil and first particle stream into a cooler, cooling and combining the heated soil and particles of the first particle stream in the cooler and discharging the combined remediated soil and particles from the cooler through a discharge, elevating the temperature of the first exhaust gas stream in a heat exchanger and passing the first exhaust gas stream at elevated temperature through a thermal oxidizer to fully destroy any residual contaminants therein, leaving a substantially clean exhaust gas stream, passing the clean exhaust gas stream from the thermal oxidizer through the heat exchanger in heat exchange relation with the first exhaust gas stream from the particle separator and for exhaust therefrom to atmosphere, combining a portion of the clean exhaust gas stream from the thermal oxidizer with residual particle-laden gases from the cooler to form a second particle-laden exhaust gas stream, flowing the second particle-laden exhaust gas stream through a second particle separator for separation into a second exhaust gas stream and a second particle stream and delivering the second particle stream to the discharge and exhausting the second exhaust gas stream to atmosphere.
In a further preferred embodiment according to the present invention, there is provided a method for remediating contaminated soils comprising the steps of heating the soil in a rotating drum to volatilize the contaminant, flowing particulate-laden gases from the drum through a particle separator for separation into an exhaust gas stream and a particle stream, passing the heated soil and particle stream into a cooler, cooling the heated soil and particles of the particle gas stream in the cooler and discharging the combined remediated soil and particles from the cooler through a discharge, elevating the temperature of the exhaust gas stream in a heat exchanger and passing the exhaust gas stream at the elevated temperature through a thermal oxidizer to fully destroy any residual contaminants therein, leaving a substantially clean exhaust gas stream, passing the clean exhaust gas stream from the thermal oxidizer through the heat exchanger in heat exchange relation with the exhaust gas stream from the particle separator and for exhaust therefrom to atmosphere, diverting a portion of the clean exhaust gas stream from the thermal oxidizer into the particle separator in heat exchange relation with the particle stream therefrom whereby the particle stream is heated and passed into said cooler and combining the clean exhaust gas stream portion and the exhaust gas stream from the particle separator prior to placing the latter in heat exchange relation in the heat exchanger.
In a further preferred embodiment according to the present invention, there is provided a method for remediating contaminated soils comprising the steps of heating the soil in a rotating drum to volatilize the contaminant, flowing particulate-laden gases from the drum through a particle separator for separation into an exhaust gas stream and a particle stream, passing the heated soil and particle stream into a cooler, cooling the heated soil and particles of the particle gas stream in the cooler and discharging the combined remediated soil and particles from the cooler through a discharge, passing the exhaust gas stream through a thermal oxidizer to fully destroy any residual contaminants therein, leaving a substantially clean exhaust gas stream, passing at least a portion of the clean exhaust gas stream from the thermal oxidizer into the particle separator in heat exchange relation with the particle stream therefrom whereby the particle stream is heated and passed into the cooler and combining the clean exhaust gas stream portion and the exhaust gas stream from the particle separator.
In a still further preferred embodiment according to the present invention, there is provided a particle separator and heat exchanger comprising means defining a chamber having an inlet for receiving particulate-laden gases and an outlet for particles separated from the particulate-laden gases, a separator in the chamber for separating the particles and gases one from the other and for directing the flow of the separated particles in a predetermined direction for flow to the outlet, means for introducing hot gases into the separator and means for flowing the hot gases in heat exchange relation with the flow of the particles whereby the particles are heated prior to their discharge from the separator.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for remediating contaminated soils in an environmentally safe manner having improved thermal and fuel efficiency whereby capital and operating costs are reduced and substantial portions of the heat in the system are recovered for use in the system.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.